I am running sendmail on FreeBSD, my e-mails are working properly. Now I want that some of my users can only mail locally / internally on that machine but that cant send mail on the internet (xxx@yahoo.com, xxx@hotmail.com etc).

If i use any 3rd party relaying server for sending emails anywhere using any email address! How he will be able to stop those emails as i am not involving his server for sending emails but receiving only!